Acanthamoeba are normally free-living protozoa found throughout the environment. Acanthamoeba are responsible for Acanthamoeba keratitis and granulomatous Acanthamoeba encephalitis (GAE) in humans (Cohen et al., 1985; Culbertson et al., 1966). Acanthamoeba keratitis is normally associated with contact lens wear and is increasingly evident in immunocompetent individuals (Auran et al., 1987; Moore et al., 1985; Stehr-Green et al., 1989).
Acanthamoeba (species) are a cause of keratitis and encephalitis in humans. Incidence of Acanthamoeba keratitis has been estimated to be 1 per 30000 contact lens wearers per year, although a separate study found that it could be as high as 20, 3.3 and 1.1 per 10,000 for extended wear, daily disposable and rigid gas permeable lenses, respectively. GAE is a disease of the immunocompromised, particularly those with AIDS where it is invariably fatal.
Current therapeutic regimens for Acanthamoeba keratitis rely on topical applications of antimicrobials every 15-60 minutes for a period of weeks making treatment arduous. These drugs include a combination of propamidine isothionate and neomycin or chlorhexidine. Current treatments are poorly effective against the cystic stages of the parasite and allow residual infection. Corneal transplantation is often necessary due to extensive damage caused by the parasites prior to diagnosis. Residual cysts remaining after treatment can result in infection of transplanted corneas (Seal, 2003). No effective antimicrobial treatment for GAE has been described although such therapy has been used with apparent effect as an adjunct to surgery (Marciano-Cabral et al., 2000; Seijo Martinez et al., 2000). This exemplifies the urgent need for new and effective antimicrobials.
The present inventors have now obtained evidence that the shikimate pathway occurs in Acanthamoeba and that inhibition of this pathway is capable of restricting the growth of the parasite in vitro. The shikimate pathway is essential for production of a plethora of aromatic compounds in plants, bacteria and fungi. The shikimate pathway was at one time believed to be absent from all protozoans, but was recently demonstrated to be present in Apicomplexan parasites. (Roberts et al., 1998).
The shikimate pathway results in the production of chorismate. In the first instance the shikimate pathway converts phosphenolpyruvate and erythrose 4-phosphate to chorismate generally in seven enzyme-catalysed steps. The shikimate pathway feeds numerous other pathways with essential compounds. In particular the shikimate pathway provides compounds for use in pathways producing ubiquinone, folate, and/or aromatic amino acids in the Amoebida. By way of an example, chorismate, produced by via the shikimate pathway, is fed into pathways for synthesising ubiquinone and folate. The synthesis of ubiquinone from chorismate depends upon eight enzymes while the production of folate in the Amoebida results from the conversion of chorismate to tetrahydrofolate in six enzymatic reactions.
In addition to the above detailed pathways, chorismate is also used in pathways, which result in the synthesis of aromatic amino acids, for example tryptophan, phenylalanine and tyrosine. Tryptophan synthesis from chorismate depends upon seven enzymes. Phenylalanine and tyrosine synthesis involve separate pathways that branch from prephenate, which is produced from chorismate via the action of chorismate mutase. The pathways that lead to either phenylalanine or tyrosine, each involve an additional five enzymes (reviewed, Roberts et al., 2002).
The molecular organisation of the shikimate pathway enzymes differs between taxonomic groups (Coggins et al., 1987). Most prokaryotes have monofunctional polypeptides encoded by separate genes, which provide seven single enzyme activities. Plants have a molecular arrangement similar to prokaryotes (Butler et al., 1974), with the exception of dehydroquinase (DHQase) and shikimate dehydrogenase, which are present as separate domains on a bifunctional protein (Mousdale et al., 1987). At least one prokaryote, Chlaniydia, also has this bifunctional fusion. All fungi examined to date have mono-functional 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthases and chorismate synthases and a pentafunctional polypeptide termed AROM (Duncan et al., 1987). The AROM polypeptide has domains analogous to the bacterial enzymes; dehydroquinate (DHQ) synthase, EPSP synthase, shikimate kinase, DHQase and shikimate dehydrogenase.
The shikimate pathway is absent from mammals, including humans that must obtain their aromatic compounds from their diet (Roberts et al., 1998; Roberts et al., 2002). This makes this pathway a potentially excellent target for new antimicrobial agents. The present inventors have found evidence that the shikimate pathway occurs in Acanthamoeba. 
As a result it is among the objects of the present invention to provide an antimicrobial compound which is effective against the Amoebida.